1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus incorporating such a fixing device.
2. Discussion of the Background
In electrophotographic image forming apparatus, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of parallel, elongated fixing members, at least one of which is heated and/or pressed against the other to define a line of contact called a fixing nip, through which a recording medium is passed under heat and pressure during the fixing process. Typical configurations of such fixing devices include a pair of cylindrical rollers, one internally heated and the other pressed against the heated one, and a combination of an internally heated cylindrical roller with a stationary member pressed against the heated roller through an endless looped belt.
FIG. 1 schematically illustrates a conventional fixing device employing an internally heated fuser roller 100 and a pressure roller 200 pressed together to form a fixing nip N therebetween.
As shown in FIG. 1, during operation, the fixing device rotates the fuser roller 100 counterclockwise and the pressure roller 200 clockwise in the drawing to feed a recording sheet S bearing a powder toner image T thereon along a sheet feed path A, which is, for example, tangent to the surfaces of the opposing rollers 100 and 200. As the sheet S enters the fixing nip N, the toner image T comes into contact with the heated surface of the fuser roller 100. At the fixing nip N, the fuser roller 100 melts toner particles with heat, while the pressure roller 200 promotes settling of the molten toner by pressing the sheet S against the fuser roller 100. The toner image T thus processed under heat and pressure then cools and solidifies and becomes fixed in place as the sheet S exits the fixing nip N to advance along the sheet feed path A.
One problem encountered by such an electrophotographic fixing device is that the recording sheet S deviates from the intended path A where the toner image T, melting and becoming tacky during fixing, adheres to the surface of the fuser roller 100 to lift, or tilt, the sheet S toward the roller 100 downstream of the fixing nip N. If the adhesion of molten toner is severe enough, it tilts a recording sheet S beyond a threshold tilt angle θ in an oblique direction B with respect to the proper sheet path A. The threshold tilt angle θ here indicates a maximum allowable tilt or deviation from the sheet feed path A with which the fixing device can separate a recording sheet S from the fuser roller 100 for forwarding it through the fixing nip N. Violating this threshold θ results in the sheet S wrapping around the fuser roller 100 to cause a jam at the fixing nip N.
To illustrate the tilt threshold in terms of a force F exerted on a recording sheet passing through the fixing nip N, proper sheet separation and forwarding occurs when the following inequality is satisfied:F1<F2where F1 represents strength of adhesion of molten toner to the surface of the fuser roller 100, and F2 represents a bending force required to tilt the recording sheet S beyond the threshold angle θ from the proper sheet path A. Typically, with the toner adhesion being fixed, using thicker and stiffer recording sheets and a fuser roller of smaller diameter results in greater threshold tilt angle θ′ and a higher bending force F2 required to pass that threshold tilt angle θ′.
To simultaneously provide both adequate fixing and smooth sheet feeding, conventional fixing devices use toner with wax or some other release agent added thereto to obtain a smaller adhesion force F1, or employ a fuser roller of a smaller diameter to obtain a higher allowable bending force F2. However, such conventional approaches remain unsuccessful where the fixing device processes thin recording sheets which are less stiff and more ready to bend than normal copy sheets. That is, using a relatively thin recording sheet means an allowable bending force F2 lower than that normally accommodated, which makes it difficult for the conventional fixing device to provide proper sheet feeding without wraparound and concomitant sheet jam at the fixing nip.
Another problem associated with an electrophotographic fixing device is the difficulty in maintaining a uniform pressure distribution throughout a fixing nip. This is particularly true where the fixing device uses a precisely cylindrical fixing roller in conjunction with an axially tapered, symmetrical fixing roller that has a diameter greatest at the center and smallest at each end (a “crowned” configuration), or conversely, greatest at each end and smallest at the center (a “bowed” configuration), which enables proper sheet feeding at relatively high speeds through the fixing nip. When juxtaposed and pressed against each other, a tapered roller and a cylindrical roller contact each other at higher pressures where the tapered roller diameter is greatest and at lower pressures where the tapered roller diameter is smallest, resulting in variation in nip pressure along the fixing nip.
It is known that variation in nip pressure translates into variation in gloss of a resulting image. That is, a printed image will be low in gloss where it is processed at relatively low pressures and high in gloss where it is processed at relatively high pressures. Such variation in gloss can detract from the appearance of the image, which is not acceptable for applications in today's high quality image forming apparatuses.
Hence, there is a need for an electrophotographic fixing device that employs a pair of fixing members defining a fixing nip therebetween, through which a recording medium can go through fixing process under a uniform pressure without wrapping around the fixing member to provide high quality printing with uniform gloss across the entire resulting image.